1. Field of the Invention
The present invention relates to insulation. More particularly, the present collapsible cellular insulation provides compact insulating sheets adapted for use with mobile and temporary shelters.
2. Description of the Related Art
Mobile and temporary shelters are an important asset for today's militaries and humanitarian relief organizations, for example. These shelters are also a common component of many civilian outdoor recreational activities. From large military structures suitable for housing aircraft to two-man pup tents, these shelters are generally easy to erect, tear down, package and transport. However, the attributes that make these shelters easy to transport also make them uncomfortable or impractical to use in extreme climates.
The roofs and walls of conventional mobile shelters are typically nothing more than panels of thin material such as canvas or nylon. These thin materials provide little thermal insulation. Therefore, to make the shelters more energy efficient, and to make them livable in extreme heat or cold, the shelters must employ insulation in combination with an environmental control unit (ECU) that supplies cooling or heat.
Thermal insulation reduces heat transfer. There are three forms of heat transfer: conduction, convection, and radiation.                Conduction: Regions with greater molecular kinetic energy pass their thermal energy to regions with less molecular energy through direct molecular collisions.        Convection: Heat conducted into a static fluid leads to a local volumetric expansion. As a result of gravity-induced pressure gradients, the expanded fluid parcel becomes buoyant and displaces, thereby transporting heat by fluid motion (i.e. convection) in addition to conduction. Such heat-induced fluid motion in initially static fluids is known as free convection. For cases where the fluid is already in motion, heat conducted into the fluid will be transported away chiefly by fluid convection. These cases, known as forced convection, require a pressure gradient to drive the fluid motion, as opposed to a gravity gradient to induce motion through buoyancy.        Radiation: All materials radiate thermal energy in amounts determined by their temperature, where the energy is carried by photons of electromagnetic energy. When temperatures are uniform, the radiative flux between objects is in equilibrium and no net thermal energy is exchanged. The balance is upset when temperatures are not uniform, and thermal energy is transported from surfaces of higher temperature to surfaces of lower temperature. The amount of energy absorption and emission by a particular object depends on the surface emissivity of that object, which is a material property. Low emissivity surfaces emit and absorb less radiation than high emissivity surfaces.        
Some types of insulation reduce conduction by providing at least one layer containing very little mass. Conduction can only be eliminated in such a layer by removing all mass from the layer, such as by providing a vacuum layer. A vacuum, however, is difficult to produce and maintain. Some types of insulation provide one or more gas-filled layers, because gases have much lower thermal conductivity than solids or liquids. The insulating ability of such an insulation can be increased by breaking up convection currents within the gas layers.
Conventional insulating materials are foam panels or fiberglass bats, for example. These types of insulation can be affixed to the shelter roof and walls to reduce the transmission of heat through the roof and walls. Unfortunately, these types of insulation are generally quite bulky. They typically occupy a volume that is many times the packaged volume of the structure with which they are used. Storage space is typically very valuable in both military and hiking/camping applications. Therefore, these conventional insulators are generally not practical for use with the majority of mobile shelters.
Large mobile military shelters typically employ ECU's. Rapid heat transfer through the roof and walls of conventional mobile shelters necessitates large capacity ECU's. These units are generally both large and heavy, making them difficult to transport and store. Additionally, ECU's require energy sources to function. Locating and/or transporting energy sources further complicates the use of ECU's with both military and civilian mobile shelters.
U.S. Pat. No. 5,270,092 (the '092 patent) provides a structural or flexible insulating panel. The panel, which may be translucent, is formed from multi-layer polymeric material in the form of an envelope surrounding a baffle. The baffle retards heat transfer across the panel using a material that forms substantially closed spaces. The closed spaces suppress convection of the low conductivity gas fill. At least a portion of the baffle carries a low emissivity surface for suppression of infrared radiation.
To function properly, the panel of the '092 patent must be inflated. This feature makes the panel unsuitable for use with mobile shelters. First, any punctures in the panel compromise the panel's insulating ability. Second, the panel requires an inflation device, which must be transported along with the panel. Such a device is likely to be large and heavy. Furthermore, if the inflation device requires an energy source to operate, the energy source must also be transported, requiring additional storage space and adding extra weight. If the inflation device is manually operated, the inflation process is likely to be time consuming and exhausting. Third, the closed cell structure complicates the packaging and repackaging of the panel.